1. Field of the Invention
The present invention is broadly concerned with novel nanoscale polymetallic composite particles, as well as magnetic recording media (e.g., flexible tapes and rigid disks) and integrated circuits using polymetallic nanoscale particles. More particularly, the invention pertains to such composite particles and end products wherein, in preferred forms, the nanoscale composite particles have an average diameter of from about 5-500 nm with an elemental metal core surrounded by a metal-containing shell material.
2. Description of the Prior Art
U.S. Pat. Nos. 4,588,708 and 4,877,647 describe catalyst and metallic coatings made by the Solvated Metal Atom Dispersion (SMAD) process. This method utilizes metal vapors (i.e., atoms) that are produced by heating pieces of elemental metal in a high temperature crucible up to the vaporization point of the metal while under vacuum. The metal vapor is condensed on the inside walls of the vacuum vessel which is cooled to a very low temperature. At the same time the vapors of an organic solvent are codeposited with the metal vapor on the low temperature vessel wall, forming a frozen matrix. After the codeposition, the frozen matrix contains metal atoms, atomic oligomers such as dimers, trimers and small metal clusters. Upon warming, the atoms and oligomers begin to migrate and bond to each other to form metal particles of various sizes, dependent upon the concentration of the metal atoms in the solvent, the chemical structure of the solvent, warm-up rate, and other parameters. Two important facets of this process are: (1) as the clusters grow they become heavier and less mobile, and (2) as the clusters grow solvent binds to the cluster surface and tends to slow further growth. Thus, the SMAD process yields metal clusters/particles in a solvent medium free of extraneous reagents.
It has also been known to employ SMAD processes for the fabrication of core/shell metallic composite particles where elemental metal particles are encapsulated within a metallic shell material. For example, metastable Fe-Mg core/shell composite particles are described by Klabunde et al. (Chem. Mater., Vol. 6, No. 6, 1994). Additionally, codeposition of Fe and Ag has been attempted, but the method failed to yield core/shell particles, resulting mainly in separate Se and Ag particles (Easom et al., Polyhedron, Vol. 13, No. 8, 1994). The key to producing metallic core/shell composites is to choose combinations of metals that are not normally thermodynamically miscible. The SMAD process forces atoms of the immiscible elements to combine at low temperatures, so that metastable alloy composite particles form. Upon heating of these particles, controlled phase segregation can be accomplished since there is a natural tendency for the two elements to separate. Although it cannot be predicted which metal will nucleate and form the core, and which will form the shell, experience has shown that the metal possessing the stronger metal-metal bonds will generally form the core material.
A major driving force behind the study of nanoscale ferromagnetic particles is the search for improved magnetic recording materials useful in magnetic recording and the like. In such applications, the ferromagnetic particles should be a single domain unit that possess two stable opposite magnetic poles along a preferred axis. The switching field is the minimum magnetic field needed to switch the magnetic poles in the single domain particles. The size of the switch units (single domain particles) is important in the performance of the recording medium. They should be small enough to allow recording of the intended magnetization pattern and to provide a high signal-to-noise ratio, which requires the use of small switch units that are partially independent, so that one unit is not strongly affected by the magnetization of the other units. Additionally, a magnetic recording medium must be chemically stable under the conditions of use. For this reason, metallic Fe, Co, or Ni, being extremely oxophilic in ultrafine particle form, are generally not useful. Instead, iron oxide, chromium oxide, barium ferrite and cobalt-enhanced iron oxide are most commonly employed. However, such metal oxides have relatively low magnetization intensities and are therefore not optimum for recording materials. On the other hand, elemental iron has a magnetization intensity of 1700 emu/cm.sup.3, which is several times that of the oxides. Accordingly, elemental iron would be admirably suited for use in recording media if the oxophilic properties thereof could be appropriately controlled.